This is not a Thunderbird issue. You will need to contact your Email Admin and give them these screenshots. Looks like they have recently implemented rDNS checks and the sending side doesn't have a proper rDNS setup for their mail server and hence email to you is bouncing. Your admin can add them to an allow list so that email can once again flow '''''OR''''' (s)he can insist that the other end fixes the missing rDNS - this is the better solution but one that you have no control over. Most email servers will give a high SPAM score for mail servers that don't have proper rDNS and ultimately lead to bounced emails such as this.